The purpose of this prospective, comparison study was to determine the accuracy of noninvasive oxygen saturation in critically ill patients. Sixteen adult critically ill patients requiring noninvasive oxygen saturation monitoring had simultaneous invasive oxygen saturation determined at periodic intervals during their ICU stay. Data were analyzed via multivariate statistics. There was a significant difference (p<.001) between noninvasive and invasive measures of oxygen saturation. We concluded that noninvasive methods may expedite oxygenation measurements but need to be used with caution in critically ill patients, especially those with pulmonary dysfunction.